shown in the nucleus. These particles are known as nucleons. The electrons are
shown outside the nucleus.
The number of protons in the nucleus of an atom determines the atomic
number. All atoms of the same element have the same number of protons and
therefore the same atomic number; atoms of different elements have different
atomic numbers. Thus, the atomic number identifies the element. An English
scientist, Henry Moseley, first determined the atomic numbers of the elements
through the use of X-rays.
The sum of the number of protons and the number of neutrons in the nucleus
is called the mass number.
Table 1 summarizes the relationships just discussed. Notice that the
outermost energy level can contain no more than eight electrons. The explanation
of this is given in the next section.
In some cases, different types of atoms of the same element have different
masses. For example, three types of hydrogen atoms are known. The most
common type of hydrogen, sometimes called protium, accounts for 99.985% of the
hydrogen atoms found on Earth. The nucleus of a protium atom contains one
proton only, and it has one electron moving about it. The second form of
hydrogen, known as deuterium, accounts for 0.015% of Earth’s hydrogen atoms.
Each deuterium atom has a nucleus containing one proton and one neutron. The
third form of hydrogen, tritium, is radioactive. It exists in very small amounts in
nature, but it can be prepared artificially. Each tritium atom contains one proton,
two neutrons, and one electron.
TIP
Isotopes have the same atomic number but a different atomic mass. This means they differ in
the number of neutrons, not protons.Protium, deuterium, and tritium are isotopes of hydrogen. Isotopes are atoms
of the same element that have different masses. The isotopes of a particular
element all have the same number of protons and electrons but different numbers
of neutrons. In all three isotopes of hydrogen, the positive charge of the single
proton is balanced by the negative charge of the electron. Most elements consist
of mixtures of isotopes. Tin, for example, has ten stable isotopes, the most of any
element.
The percentage of each isotope in the naturally occurring element on Earth is
nearly always the same, no matter where the element is found. The percentage at
which each of an element’s isotopes occurs in nature is taken into account when
calculating the element’s average atomic mass. Average atomic mass is the
weighted average of the atomic masses of the naturally occurring isotopes of an